Incoherent C-mode ultrasound detectors have been used for high resolution surface and contact imaging in scanned single element devices, similar to acoustic microscopes, and also 2D arrays. However, all of these detectors use a pulse-echo methodology to collect reflectivity information from the surface of interest. The complexity of the electronics and acoustic structures needed to scale these devices to large areas becomes complex, impractical, or expensive.
Techniques for large-area ultrasound contact imaging have a number of disadvantages. High resolution, C-mode, contact/surface images may be acquired by scanning a highly focused ultrasound transducer over a surface, similar to an acoustic microscope. The focused transducer operates in pulse-echo mode, in which an ultrasound pulse or tone burst is transmitted to the surface and the received reflected response is peak detected at the appropriate time/range gate associated with the surface. Since scanning a single transducer is very time consuming, there has been development of large-area 2D arrays that collect a whole image plane synchronously. However, because the surface of interest is typically a large distance, greater than many wavelengths, away from the receiver face, large apertures are needed to maintain high resolution at the imaging surface. In the single element transducer system, this means that the scan step size and resolution at the focal point is often much smaller than the diameter of the single-element transducer. Thus, simply tiling single element transducers to produce a large-area array is not only difficult, but also will not provide the pixel density needed for large-area, 2D, ultrasound contact imaging.
U.S. Pat. No. 5,456,256 and U.S. Patent Application Number 2007/0258628 A1 describe two-dimensional (2D) array approaches that use dynamic focusing of smaller-sized elements. However, the electronics required to time delay and phase each element are complex and difficult to match over a large number of elements. This approach also requires the distribution of common reference signals, which is a challenge over a large area. Because of the complexity of these circuits, they prohibit the use of polysilicon thin film transistors (TFTs) and other low-cost, large-area electronics. Thus, the electronics requirements for this approach are prohibitively complex and expensive when scaled to large-areas.
Acoustic structures, such as lenses and waveguides, may be used to maintain high resolution for pulse-echo contact imaging with incoherent arrays. U.S. Pat. Nos. 5,483,963 and 6,552,841 describe large lenses used to focus ultrasound from the image plane onto an incoherent focal plane detector. In this approach, the resolution is limited by the aperture of the lens, which is not easily scalable to arbitrarily large areas. In addition, the lens structure requires large separations between the focal plane array, the lens, and the image plane; this limits the overall size of the final device.
U.S. Patent Application Numbers 2008/0258580 A1 and 2008/0219098 describe waveguide structures used to prevent ultrasound spreading to maintain resolution at the image plane. However, this structure suffers from difficulties in manufacturing and performance. To achieve the density and resolution needed, the acoustic waveguides often need to be spaced very close together (on the order of or smaller than a wavelength). Because the cores are spaced so closely together, the cladding layers are very thin and allow crosstalk between neighboring cores that degrades lateral resolution. In addition, fabricating the waveguide plates and aligning them with an array becomes increasingly difficult for large areas.
Pulse-echo structures and methods of ultrasound contact imaging are not scalable to large-area, incoherent devices capable of high-resolution, low-cost imaging.